1. Field of the Invention
The present invention relates to an apparatus and method for finding the location of the inside wall of a transparent or translucent tube for the purpose of measuring the bore. It also concerns the relationship between light from a laser beam aimed normal to the axis of the tube and toward the inside wall and the consideration of polarized rays reflected and refracted by the inside wall.
2. Background Description
It is important to be able to measure the inside diameter of a transparent or translucent tube in order to manufacture tubing with a consistent bore. It is particularly important that measurements be made with great accuracy and precision in order that the bore diameter be a consistent dimension along the axis of the tube so that the tube bore is uniform. Uniform diameter provides better control of volume inside the tube.
U.S. Pat. No. 3,994,599 shows a method and apparatus for measuring the wall thickness and concentricity of tubular glass articles. The patented method and apparatus uses a beam of coherent light directed toward the tubular glass article so that when portions of the beam are reflected by the inner and outer tube surfaces, the reflected portions converge at a point spaced from the article and the spacing is related to the wall thickness. When a plurality of such beams is directed at circumferentially spaced points, interference fringes are produced and the spacing of the fringes varies at each of the points when there is a lack of concentricity between the outer and inner surfaces of the tube. The laser beam reflections of that patent are used to measure wall thickness and not the bore diameter of the tube. Varying wall thickness is a measure of concentricity and not of the bore diameter at any point or at points along the axis.
U.S. Pat. No. 3,307,446 shows a light source and photo pickup used to gauge the wall thickness of glass tubing as the tubing is being drawn. Again the differences in light reflections from the inner and outer walls of the tube are measured relative to one another to give the wall thickness. This technique is based on the triangulation principle and therefore the resolution of the measurement is limited by the spatial resolution of the detector array and the included angle between light source and detector. The light signals received in this technique are weak, being only interface reflections, and therefore the possibility of interfering signals is high. Also, this technique is not independent of index of refraction changes in the tube being measured. It has also been known to measure holes using light sources and reflection; see, for example, U.S. Pat. Nos. 3,806,252 and 4,690,556. The latter patent shows a method for checking the straightness of an elongated generally cylindrical bore by directing a collimated light beam along the bore with the bore axis skewed slightly with respect to the beam center line. The wall of the bore reflects some of the light beam forming generally an off-axis ring of reflected light beyond the end of the bore and an on-axis spot of unreflected light. Scatter of the reflected light indicates non-straightness of the reflecting bore wall. Rotation of the cylindrical bore through a plurality of positions permits checking the entire circumference.
While that method can be used to check straightness of the bore, diameter and out of roundness, the patented apparatus and method cannot be used for gauging diameter of tubing as it is being drawn as that method requires the laser beam to be directed axially through the bore but skewed slightly relative to the axis.
Measurement of the diameter of a transparent tube during manufacture of the tube is an important factor in the drawing of tubes used for scientific purposes. An apparatus for drawing precision glass tubing from molten glass is disclosed in U.S. Pat. No. 3,401,028. There the glass is heated to a flowable and workable condition for drawing. Prior to cooling the dimensional characteristics of the drawn tube are affected by a device inside the tube which controls the internal dimensions of the tube. The apparatus is used to produce high-precision tubing with uniform dimensions held to diametrical variations of less than 0.5% and preferably of the order of 0.1% or less.
One example of how a precision bore glass capillary tube can be used for scientific purposes is the analysis of blood samples in a quantitative buffy coat centrifugal analyzer. QBC.RTM. is a product sold by Becton Dickinson and Company, Franklin Lakes, N.J. and it includes a capillary tube containing a solid cylindrical plastic float. A patient's blood is drawn into the tube, the tube end is sealed with a plastic cap and the tube spun in a microhematocrit centrifuge for five minutes. During centrifugation the plastic float having a specific gravity that is midway between that of the plasma and the red blood cells floats on top of the red blood cells and is surrounded by the expanded buffy coat. The float occupies more than 90% of the cross-sectional area of the tube and so the buffy coat is expanded ten-fold in the space between the inside wall of the tube and the outside diameter of the float. The individual buffy coat layers can be easily measured. U.S. Pat. Nos. 4,567,754 and 4,190,328 disclose quantitative buffy coat tubes and the background in each patent is instructive on the procedures used to read buffy coat.
The fit between the bore of the capillary tube and the float is important in that particular application. Therefore, the manufacture of the capillary tubing with a precision bore is a primary concern. In the past such tubing manufacture had to rely on a variety of techniques to measure the accuracy of the tubing bore diameter, usually of the finished product. During manufacture the dimensions of the capillary tubing and its respective float had to be checked with a standard calibrating liquid to see that the spread of material captured between the inside wall of the tubing and the outside of the float was as expected. This procedure is cumbersome, slow and difficult to perform accurately. It is of value to be able to use a light source such as provided by a laser, to give an accurate reading of the inside diameter of a precision bore capillary during the tube drawing process.